The murderer who took the lives of 17 people at a high school in Parkland, Fla., was on everybody’s radar, from the school authorities to the local police to the FBI, which failed to follow its own protocols when a person close to the future killer called to warn the bureau that he was contemplating a school massacre. Everybody knew, nobody did anything.

What should be done?

Gun control, the Left says, always, offering ideas that range from the trivial to the patently unconstitutional. Whatever the outrage of the day is, the Left’s answer is to seek to ban whatever implement was used in it: Until the murders in Las Vegas, nobody outside of gun-nut circles had heard of a bump stock, but bump stocks became Public Enemy No. 1. In the debate over so-called assault weapons, every feature from the functional to the cosmetic has had its turn as the deification of evil.

The Parkland killer didn’t use some exotic weapon. He used a ubiquitous .223-caliber AR-pattern rifle, the most common sporting and target-shooting rifle in the United States. Attempts to mythologize the AR as an exotic weapon of war are reliably absurd. Dr. Ernest E Moore, writing in Time, displays the usual ignorant fear-mongering about the AR: “An assault rifle is designed to deliver fatal wounds to multiple individuals within a short time period; it has no other purpose,” he writes. “It’s unclear what other purpose it could serve, given how and why it was made.” In reality, AR-style rifles are used in everything from small-game hunting to competitive marksmanship. Dr. Moore and other gun-control advocates reliably elide the differences, but the civilian AR rifle is a very different instrument from its fully automatic cousins issued to U.S. armed forces.

What kind of gun control might have prevented the Florida killing? For all the talk of “moderate” and “common-sense” reforms, plausible regulatory approaches to preventing such acts of mass violence are few and far between. The Florida shooter had no criminal record and had not been judged mentally incompetent, and so he was able to purchase rifles and other long guns legally, passing a background check in the process. Given the fact that the local police and the FBI both failed to look seriously into him after receiving credible warnings of his murderous intent, it is difficult to imagine that a clerk at a sporting-goods store is going to be much more effective staging an intervention at the point of sale.

Some gun-control advocates would like to ban the sale of semiautomatic rifles and handguns — which is to say, the majority of rifles and the great majority of handguns — but there already are millions of those in circulation. These would have to be confiscated (as they were in Australia) in order to make such a program effective. The prohibition of most small arms and the seizure of millions of firearms already in private hands might be a lot of things, but it is not “moderate” or “common sense.” It would be a radical step, and one that almost certainly would be found to be unconstitutional.

We already have in place protocols for placing people in temporary custody when they are judged likely to endanger to themselves or others. But we cannot make use of those powers if law-enforcement and mental-health authorities won’t intervene.

All long guns combined — from granddad’s duck gun to the scary-looking black instruments that so repulse our progressive friends — represent a vanishingly small share of the weapons used in violent crimes in the United States, something on the order of 2 percent in a typical year. In spite of the media hysteria, violent crimes involving so-called assault rifles are so rare that the FBI data don’t even break them out into a separate category.

There are really two separate questions at work here, one involving ordinary crime and the other involving public spectacles like the one in Florida. When it comes to workaday violent crime, there is room for meaningful gun control — which means prosecuting violent criminals and their enablers rather than harassing hobbyists down at the local gun store. In city after city, the data show that homicides overwhelmingly are committed by people with prior police records, often involving violent crime committed with firearms. In New York City, about 90 percent of the homicides are committed by people with prior records, according to a New York Times survey of police records. Like the killer in Florida, they don’t just come out of nowhere. Dealing with that means most vigorously prosecuting violent crimes short of homicide as well as doing the very hard work of improving the parole and probation systems. It also means following up on prosecutions in straw-buyer cases, something that both local and federal authorities have neglected to do — often as a matter of policy, as in the case of the U.S. attorney’s office in Chicago. Forgive the shopworn phrase, but we really ought to at least try enforcing the laws on the books before we go passing burdensome new ones.

As for homicidal spectaculars like the one in Florida, additional gun-control measures are unlikely to prove effective. (It’s worth keeping in mind that two of the worst of those massacres — at Bath, Mich., and Oklahoma City — involved no firearms at all.) But that does not mean that nothing can be done. Our police and mental-health authorities, including the fumbling FBI, are not performing as well as they should be. Many of these cases could have been prevented well before the bullets started flying. Perhaps the gentlemen at the FBI could be persuaded to pick up a phone from time to time in cases such as these. Omar Mateen, Nidal Hasan, Adam Lanza, Sayfullo Saipov — none of these men simply stepped out of the shadows. We already have in place protocols for placing people in temporary custody when they are judged likely to be a danger to themselves or others. But we cannot make use of those powers if the relevant law-enforcement and mental-health authorities are unable or unwilling to intervene.

Beyond that, defensive measures are called for. Schools are targets and we should treat them as such, with better gatekeeping and, if necessary, armed guards. We wish it were not the case, but wishful thinking isn’t enough. We also can do more with venue security, especially for wide-open events such as that concert in Las Vegas.

There are many distinct kinds of violent crime. The drug murders on Chicago street corners are phenomena distinct from the killings in high schools and other public places, with different patterns of behavior, different kinds of perpetrators, different kinds of weapons, different motives, etc. The desire for a simple — “common-sense” — solution to these horrifying crimes is easy to understand. But it is stupid and dishonest to pretend that changing the rules for private transactions at gun shows is going to stop that, or that similar measures are likely to have any meaningful effect at all. Policing criminals is harder work than policing the legal commerce of federally licensed firearms dealers, but it is the work that we are paying for, and the work that we need done.